There are two commercial coffee species, Coffea arabica L. (arabica type coffee) and Coffea canephora p. ex Fr. (canephora or Robusta type coffee). Coffee is a tropical and perennial tree which produces coffee berries. The coffee beverage is produced by percolating hot water through roasted coffee seeds. Arabica coffee provides a superior coffee beverage and it is typical of the highland growing regions while Robusta coffee produces a lower quality coffee and is grown in lowland regions.
A breeding cycle for coffee plants takes four years, i.e., seeds to a flowering plant and back to seeds. Selections for yield are usually made from 6-8 year old coffee plants. Several cycles of crossings and selections are required until one finds superior genotypes. In order to assure fidelity through seed propagation, six cycles of selfing are required. Considering one cycle for hybridization and selection and six cycles for seed homozygosity, a normal coffee breeding program requires 28 years. Cell culture techniques, such as somatic embryogenesis, clonal propagation, and protoplast regeneration provide opportunities to shorten the time requirement for coffee improvement.
The pioneer cell culture work on coffee was reported by Staritsky, Acta Bot. Neerl. 19:509-514, 1970, who described the induction of somatic embryos and plantlets from orthotropic shoots of C. canephora. Subsequently, Colona, Cafe Cacao The 6:193-203, 1972, established cultures of embryos of C. canephora and Coffea dewevrei. Pursuant to the study of caffeine synthesis, Keller et al. Planta 108:339-350, 1972, induced callus from endosperm tissues of C. arabica. Sharp et al. Phyton 31:67-74, 1973, cultured somatic and haploid tissues of C. arabica. obtaining callus growth (petioles, leaves, green fruit), proembryo formation (anthers) and shoot development (orthotropic shoots). Induction of abundant friable callus from perisperm tissues of C. arabica and Coffea stenophylla was reported by Monaco et al. Applied and Fundamental Aspects of Plant Cell, Tissue, and Organ Culture, J. Reinert and Y. P. S. Bajaj, eds., pp 109-129, 1977 Springer-Verlag, Berlin. Furthermore, perisperm proliferated rapidly in the absence of auxin, suggesting that it was autonomous for auxin. Liquid cell cultures of C. arabica cv. El Salvador derived from orthotropic shoots were reported by Townsley, Can. Inst. Food Sci. Technol. J. 7:79-81, 1974. These same suspension cultures were used to analyze caffeine and chlorogenic acid contents by Buckland and Townsley, J. Inst. Can. Sci. Technol. Aliment 8:164-165, 1975, and to compare unsaponifiable lipids in green beans with those in cell suspensions as reported by van der Voort and Townsley, J. Inst. Can. Sci. Technol. Aliment 8:199-201, 1975. Subsequently, Sondahl and Sharp, Z. Pflanzenphysiol. 81:395-408, 1977; Plant Cell and Tissue Culture: Principles and Applications, W. R. Sharp, P. O. Larsen, E. F. Paddock and V. Raghavan, eds., pp 527-584, 1979, Ohio State Univ. Press, Columbus, distinguished between high and low frequencies of somatic embryo induction from cultured mature leaf explants of C. arabica cv. Bourbon. A histological study demonstrated the somatic embryos from coffee leaf explants were derived from mesophyll cells. Furthermore, low frequency somatic embryos (LFSE) were present as early as 70 days whereas embryogenic tissue and high frequency somatic embryos (HFSE) were detected only 90-120 days of secondary culture (Sondahl et al. Z. Pflanzenphysiol. 94:101-108, 1979). HFSEs were derived from a distinct cellular phenotype (small and round-shaped, ca. 20 mm diameter) in contrast with callus cells (long rods, ca. 150 mm long). Mature leaf cultures were capable of yielding HFSE.
In summary, plant regeneration from coffee tissue has been accomplished from solid cultures of orthotropic shoots and mature leaves covering four species of the Coffea and five C. arabica cultivars. More particularly, direct (LFSE) and indirect (HFSE) somatic embryogenesis routes are frequently associated with coffee tissues together with shoot development.
Cloning of selected coffee plants can be accomplished through nodal cultures or by development of embryogenic cell lines in liquid phase using bioreactor techniques. Characteristically, coffee plants have ten arrested orthotropic buds and two plagiotropic buds at each node of a coffee stem. The plagiotropic buds differentiate only after the 10th-11th node of a developing seedling, whereas the orthotropic buds are present beginning with the first node (cotyledonary node). The excision and culture of the uppermost nodes permits the recovery of cloned coffee plants from each orthotropic bud. These plants are clones of the donor plant. The technique to culture these nodal tissues and recover normal coffee plants from these axillary buds permits the application of this methodology for vegetative propagation of coffee plants. The advantage of bioreactor cloning is the utilization of embryogenic cells in a liquid medium and the recovery of a very large number of plants within a short period of time. For example, a one liter bioreactor could yield ca. 250,000 plantlets.
Somaclonal variation refers to the genetic variability found among plants derived from in vitro cultures of somatic cells. Variation in cultivated plant cells has been observed since the beginning of plant tissue culture. This variability was associated with chromosomal abnormalities due to long-term cell culture. Later, the recovery of plants with chromosome variation was reported in sugar cane and potato. It was then proposed as a novel source of agriculturally useful variation for crops. More recently, evidence has been presented that variant characters expressed in plants derived from the culture of somatic tissues are transmitted to the progeny. These results open the door for the use of cell culture as a tool to recover stable variation in any plant species.
Presently, somaclonal variation has been described in at least 16 plant species: tomato, potato, tobacco, rapeseed, celery, lettuce, garlic, pineapple, sugar cane, wheat, rice, corn, oat, barley, flax and sorghum. In the case of tomato, such variability has been genetically analyzed during the R.sub.0, R.sub.1 and R.sub.2 generations. Chromosomal changes, as well as nuclear and cytoplasmic mutations, have been described. The origin of somaclonal variation can be explained by (a) chromosomal changes (rearrangements, deletions, number); (b) nuclear mutations; (c) organelle mutations; (d) mitotic crossing-over; and (e) cell sorting. Genetic analysis of nuclear mutations of somaclones have demonstrated several characteristics to be monogenic.
The events leading to somaclonal variation can take place during the ontogeny of the tissue (preexisting variation) or during the cell culture period (in vitro variation). The morphogenetic process of plant regeneration (organogenesis vs. embryogenesis), explant sources, genotypes and culture conditions are all important factors in determining the frequency of somaclonal variation.
Practical advantages of using somaclonal variation techniques for plant breeding resides in the fact that the variability may be associated with single gene mutations of somatic cells of commercial cultivars. As a consequence of this characteristic, the new variants are almost identical to the mother plant except for the new trait. Following two cycles of field testing to insure the stability of the new trait and yield performance of the selected somaclone lines, new plant varieties can be released within a shorter time than required by conventional breeding. This fact can be advantageous to plant breeders since it avoids the long cycles of selfings and backcrossings in order to eliminate the undesirable genes introduced during the original cross with the wild type. Another advantage to plant breeding is the access to new nuclear mutations and also to organelle mutations (genes coded in the cytoplasm) within a commercial genotype.
In a crop improvement program somaclonal variation methodology has to interface with conventional breeding. One obvious strategy is to introduce the best available varieties into cell culture and select for improved traits. Somaclonal variation can be used to uncover single gene mutations that retain all the favorable qualities of an existing variety. Several in vitro cycles can be used to further improve one particular genotype, (i.e., second, third, etc. generation of somaclones).
Coffea arabica cultivars were derived from a very narrow genetic reservoir. Although hybridization has been the primary method of developing new breeding lines, induced mutations could offer an opportunity to increase the range of genetic variability for the breeder. However, the data currently available indicate that cuttings, germinating seeds, and pollen grains of C. arabica are quite resistant to ionizing radiation such as gamma and x-rays (2-100 krad) and to chemical mutagens. Although a very large number of samples have been irradiated, no identifiable mutants have been isolated. Treatment of seeds with recognized powerful mutagens, e.g., EMS or sodium azide, have failed to produce a single distinct mutation despite examination of several thousands of plants. Somaclonal variation therefore, offers an excellent opportunity for developing unique types of genetic variability for the selection of new breeding lines of coffee plants. Furthermore, since somaclonal variants are derived from spontaneous mutations (nuclear or cytoplasmic), it will be possible to release new varieties in shorter periods of time provided that the cultures were derived from tissues of superior commercial coffee genotypes.
For example, Laurina is a natural mutant of C. arabica that retains the good Arabica taste but has only 50% of the Arabica caffeine content (0.7%), a highly desirable trait. Despite the fact that the Laurina mutation has been known since the late 1800's, Laurina plants have not been commercially viable due to low bean yields and susceptibility to diseases, particularly coffee leaf rust. Laurina arose as a natural mutation isolated from the Arabica coffee variety Bourbon and was first discovered at Reunion Island (21.degree. latitude South), Africa. Laurina is also called "Bourbon Pointu" or "Ismirna coffee". The Laurina mutation has been characterized as a single recessive gene mutation (lr lr) with a pleiotropic effect on caffeine content (0.6-0.7%) and plant morphology (very small leaves, short internodes, short plant height, elongated fruits). Hence, Laurina has about a 50% reduction in caffeine content relative to Arabica varieties. Until the early 1990's, attempts were made to cultivate Laurina in small farms due to its good beverage quality, but were completely abandoned because of low yield. Consequently, Laurina somaclones which produce high an yield and have the naturally low caffeine content (or less) typical of the Laurina variety are highly desirable given the demand for decreasing the caffeine content of coffee products.
This invention, therefore, fulfills a long felt need to develop coffee variants with improved processing and agronomic characteristics and in particular, to develop methodologies to generate said variants in coffee species.
To date, somaclonal variants in coffee have not been reported. Genetic variation in coffee is considered rare due to its tetraploid nature. Additionally, appropriate tissue culture and field trial conditions have not been well established for coffee. Certain tissue culture conditions have been reported by Frischkneckt et al., Handbook of plant Cell Culture 3:564-590, 1984, Ammirate et al. Eds, Macmillan Press, New York, London. The use of biotechnology for coffee improvement is also discussed by Baumann, Association Scientifique Internationale du Cafe', Colloque, Lone', 55-68, 1985. Tissue culture conditions are also discussed by de Menendez, Cafe Cacao The 31:15-22, 1987.